An “Uncertainty Principle” for the Mediterranean annual dry grasslands

Abstract

Preliminary results of diachronic researches on the spatial arrangement and species richness in Mediterranean annual dry grasslands (Tuberarietea guttatae) demonstrated that the patchiness resulted minimized and the temporal stability maximized at elevations offering the best compromise between the summer drought stress and the winter cold stress. Moreover, the temporal stability of the Mediterranean annul dry grasslands could not necessarily be related with their demographic inertia, meaning that the turnover and rearrangement of species within the community do not necessarily implicate significant changes in the average species composition over time. On the other hand, the survival chances of Mediterranean annual dry grasslands are enhanced by disturbances, such as higher temperatures, periodical fires, seed rearrangement and predation by ants. The influence of stochastic disturbances, acting together with year-to-year climatic fluctuations that are typical of the Mediterranean climate, drives the Mediterranean annual dry grasslands to acquire a dynamic long-term flow equilibrium, defined as homeorhesis (“preserving the flow”), that in thermodynamic terms can be seen as a system depending on the existence of dissipative structures (perturbations) producing high internal entropy that impede to the system to achieve an homeostatic equilibrium. We can assume that the Mediterranean annual dry vegetation consists of chaotic assemblages of species constrained within certain limits of predictability by the spatial heterogeneity and stochastic disturbance. In particular, frequent disturbances of low intensity do increase the species richness and unpredictability of a Mediterranean annual dry grassland, but under such conditions its spatial patchiness is minimized and its temporal stability maximized (so that the distribution pattern of species is more predictable and homogeneous). On the other hand, intense disturbances, although less frequent, decrease the species richness of a Mediterranean annual dry grassland and makes it more predictable, but the species assemblage and temporal stability undergo greater fluctuation. These considerations lead to postulate a “uncertainty principle” for the Mediterranean dry grasslands: if the spatial pattern is predictable, the species composition is largely unpredictable and fortuitous; if the species composition is predictable, the spatial pattern and temporal stability are largely unpredictable and changeable.